A variety of kinetic and spectroscopic methods are utilized to characterize the mechanism of electron transfer in important biological systems. These systems are chosen to provide a broad cross-section of complexity ranging from soluble proteins with single redox centers and soluble proteins with multiple redox centers to the membrane bound redox systems of mitochondria. By adopting a program which is broad both in methodology and in biological material, I hope to develop an unusually powerful attack on this fundamental biological problem. The principal techniques, optical and magnetic resonance spectroscopy, will be employed in both equilibrium and kinetic experiments so that the pathways of electron transfer might be elucidated. Simple electron transfer proteins such as cytochrome c and the ferredoxins are expected to provide insights into more complex soluble enzymes, e.g., xanthine oxidase, which in turn may serve as a prototype for examining more complicated enzymes of membraneous origin such as cytochrome oxidase.